mmc: tegra: implement memcomp pad calibration

The Tegra30+ SDMMC module has memcomp pads that are used to
automatically find and set the correct drive strength settings to
the sdmmc pads. The calibration needs to be manually kicked off
when the card signal voltage is changed, after the card clock is
supplied again.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
[Ulf: Rebased to fix a trivial compile error]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>